The Micromeritics Pycnometer 1305 allows the determination of the real volume of a porous material (i.e. the volume of the solid, excluding pore volume) and thus its real density. A certain mass of solid is placed in the sample cell whose calibrated volume is precisely known. The sample cell communicates with an expansion chamber whose volume is calibrated as well. A certain pressure of helium is introduced in the sample cell where it fills up the whole volume free of solid, including the porosity. The sample cell and the expansion chamber are then put into communication and the pressure after expansion is measured. The sample volume is calculated through this pressure drop caused by the gas expansion.